1. Field of the Invention
The present invention relates to a method and apparatus for optical pickup, and more particularly to a method and apparatus for optical pickup that is capable of performing an effective polarization split.
2. Discussion of the Background
As a variety of optical recording media such as a CD (compact disc), a DVD (digital versatile disc), etc. are developed, research and development of suitable optical pickup devices becomes crucial. In general, an optical pickup device for reading to and writing from an optical recording media includes a light source emitting laser light, an optical system including lenses and so on for irradiating an optical recording medium with the laser light emitted from the light source, an optical detection mechanism for detecting reflection laser light reflected by the optical recording medium, and so forth. Such an optical pickup device is typically provided with a polarization split device. The polarization split device allows the laser light directed towards the optical recording medium from the light source to pass entirely therethrough, and causes the laser light to diffract relative to the reflection laser light reflected by the optical recording medium so that the laser light is split. This method uses a light path through which the laser light is caused to travel forward and backward. Such a polarization split device generally includes diffraction gratings having different optical characteristics, e.g., having different transmittance, diffraction efficiency, etc., according to states of a polarization plane of the laser light.
Also, the optical pickup device is generally provided with a quarter-λ plate (also referred to as a quarter frequency plate) at an object lens side of the polarization split device. With this quarter-λ plate, reflected laser light having a polarization direction rotated by 90 degrees is caused to impinge on the polarization split device so that the reflected laser light is guided to a light detecting device.
In recent years, research and development have been increasingly made for a single optical pickup device that is capable of reading from and writing to a plurality of different optical recording media such as a CD (compact disk) and a DVD (digital versatile disk), for example, having different recording densities.
To read from and write to a plurality of different optical recording media, such as a CD and a DVD, for example, having different recording densities with a single optical pickup device, it is necessary to provide the optical pickup device with two laser light sources having different wave lengths.
When a single optical pickup device with two laser light sources having different wave lengths is provided with two different optical systems for the two laser sources, the size of the optical pickup device becomes large, which may be a problem. To avoid this problem, an optical pickup device having two different laser light sources and a single optical system has been developed. In this development, various components used in the two different optical systems are necessarily made into common components.
The above-described polarization split device is one of the components needed to be modified into common components. One example of the polarization split device made as a common component is described in published Japanese unexamined patent application No. JP11-311709, in which a polarization split device is fabricated based on an LN (lithium niobic acid) substrate with an ion-exchange process. In the ion-exchange process, however, splitting of areas depends on dispersion, and therefore it is very difficult to conduct splitting in a perfect manner.
Another example is described in published Japanese unexamined patent application No. JP11-295510, in which a polarization split device is fabricated based on a diacetylene monomer film to which ultraviolet rays are irradiated. In this case, perfect parallel ultraviolet rays are required to carry out splitting of areas. In addition, reflection light rays from the substrate, scattering light, and the like, may degrade allowable operating conditions. Therefore, it may be very difficult to split every uneven portion in an ideal manner.
Another example is described in published Japanese unexamined patent application No. JP11-064615, in which a polarization split device is fabricated such that patterns are formed with a dry etching process in a high polymer liquid crystal film formed on a glass substance and are then charged with a filler having a refractive index that matches the high polymer liquid crystal film. In the dry etching, however, a side etching or a mask recess may cause an adverse effect on the shape of a diffraction grating, and therefore it may be vary difficult to form a diffraction grating in a perfect rectangular shape.
When a diffraction grating formed in an imperfect rectangular shape is used, even-numbered-order diffraction light, which should not be generated from a design point of view, may be generated and cause reduction of efficiency as well as generation of undesired light. As a result, an adverse effect on a light detection may occur. This leads to an erroneous reading and writing of the optical pickup device relative to an optical recording medium.